1. Field of the Invention
The invention relates to an image forming apparatus using an electrophotographic printing method or an electrostatic recording method such as, for example, copying machine, a printer or a facsimile apparatus, and particularly to an image forming apparatus adapted to form a visible image, i.e., a developed image (toner image), on an image bearing member by the electrophotographic printing method, the electrostatic recording method or the like, and transfer the toner image to a recording material through an intermediate transfer member.
2. Related Background Art
In recent years, as a plural-color or full-color image forming apparatus adopting an electrostatic process such as the electrophotographic printing method or the electrostatic recording method, there has been proposed an image forming apparatus of a so-called intermediate transfer type which successively superposes toner images of respective colors formed on a photosensitive drum which is an image bearing member on an intermediate transfer member to thereby form a color image, and collectively transfers the color image to a recording material.
In this intermediate transfer type, a toner image is formed on the photosensitive drum by charging means, exposing means and developing means disposed around the photosensitive drum, and in a primary transferring portion, the toner image is electrostatically transferred to an intermediate transfer belt as the intermediate transfer member by transferring means. When a color image is to be formed, toner images are successively transferred to the intermediate transfer belt, whereby a full-color image can be formed on the intermediate transfer belt (image bearing member).
The toner images transferred to the intermediate transfer belt are conveyed to a secondary transferring portion by the rotation of the intermediate transfer belt, and are electrostatically transferred to a recording material. As a method of removing any toners not transferred to the recording material at this time, but residual on the intermediate transfer belt, there has been proposed a method of pushing a cleaning blade against the intermediate transfer belt to thereby remove the residual toners, or a method of applying a bias to fur brush cleaning means to thereby electrostatically remove the residual toners.
The fur brush cleaning is advantageous to such a problem as the influence upon the life of the intermediate transfer belt which poses a problem in blade cleaning means or a load fluctuation due to the fluctuation of frictional resistance, but the untransferred toners after the secondary transfer include toners charged to the plus (+) polarity and toners charged to the minus (−) polarity, by a secondary transfer bias, and therefore there arises the problem that all of the untransferred toners cannot be collected by a single fur brush and by the application of a bias of one polarity.
Against this problem, Japanese Patent Application Laid-open No. 2002-207403 discloses a method of applying a plus (+) bias and a minus (−) bias differing in polarity to a plurality of fur brushes (cleaning means) from a power supply (bias applying means) to thereby collect the untransferred toners after the secondary transfer.
Also, as the control of an electrostatic cleaning type, as described in Japanese Patent Application Laid-open No. H4-178680, there has been proposed a method of controlling a bias voltage applied to cleaning means depending on the ambient environmental condition.
Further, Japanese Patent Application Laid-open No. H4-251276 proposes a method of controlling a transfer bias applied to transferring means depending on the impedance of a recording material in a secondary transferring portion.
In the above-described image forming apparatus, however, there has arisen the problem that depending on the types of the recording material, the states of the toners not transferred to the recording material but residual on the intermediate transfer member differ, and suitable cleaning of the intermediate transfer member is not done.
That is, the methods described in the Japanese Patent Application Laid-open No. H4-251276 alleviate the unevenness of a cleaning property due to the environment, but even in the same environment, depending on the types of the recording material, unevenness occurs to the amount of untransferred toner to be removed, and this may sometimes cause the problem of unfaulty cleaning. Particularly, an embossed recording material greatly differs in the amount of untransferred toner depending on the irregularity (indentation and salient) of the surface thereof, and in one cycle of image formation, and in the longitudinal direction of the belt, there are mixedly present portions great in the amount of untransferred toner and portions small in the amount of untransferred toner, and this is liable to cause faulty cleaning in the portions great in the amount of untransferred toner.
Also, according to the method described in the Japanese Patent Application Laid-open No. H4-251276, the transfer bias can be controlled by the impedance of the recording material, but as described above, even during one cycle of secondary transfer, the close contact property in the secondary transferring portion differs depending on the salient and indentation of the surface of the recording material and therefore, a proper transfer bias conforming to the indentation and salient of the paper cannot be selected.
FIG. 8 of the accompanying drawings is a graph showing the density of image relative to the transfer voltages in an indentation and a salient when a solid image has been transferred to embossed paper.
As shown in FIG. 8, on the embossed paper, between the indentation and the salient, a difference occurs to transfer pressure in a secondary transferring nip (secondary transferring portion) and therefore, the transfer characteristic differs. Further, in the indentation, a minute clearance occurs in some cases, and this leads to the occurrence of a case where the transfer efficiency itself becomes low. Therefore, if the transfer voltage is adjusted to the transfer property of the salient, the amount of untransferred toner residual on the transfer belt will become great in the indentation. If conversely, the transfer voltage is adjusted to the transfer property of the indentation, the amount of untransferred toner residual on the transfer belt will become great in the salient.
In contrast, in the electrostatic fur brush cleaning, when for example, the charging polarity of the toner is minus (−), the toner collecting capability can be raised by making the bias applied to the fur brush opposite, i.e. plus (+) in polarity to the toner, and increasing the intensity of the bias, but if the bias continues to be applied with its intensity increased, the toner deposited in the fur brush will be charged to the plus (;) polarity by charge injection or discharge, and there will arise the problem that the once collected toner is discharged onto the intermediate transfer belt. The toner thus discharged onto the belt is transferred to the recording material during the next image formation and causes a faulty image.
Therefore, the lower limit value of the bias setting of the electrostatic fur brush cleaning is set to bias intensity which can collect the untransferred toner, and the upper limit value thereof is set to bias intensity at which it is difficult for the toner to be reversed during collection. As the result, an upper limit is formed in the collecting capability by the upper limit value of the bias intensity.
FIG. 9 of the accompanying drawings is a graph showing the relation between the amount of secondary-transferring residual toner and the electrostatic cleaning bias. An area A in FIG. 9 is an area in which the slipping-out of the toner occurs because an amount of secondary-transferring residual toner exceeding the cleaning capability is carried to this area, and an area B is an area in which the bias intensity is too high and the toner deposited in the fur brush begins to be discharged onto the intermediate transfer belt.
That is, when an attempt is made to prevent the discharge, the amount of secondary-transferring residual toner X mg/cm2 in FIG. 9 assumes the upper limit value of fur brush cleaning.
However, in a case where the amount of secondary-transferring residual toner in the indentation of the embossed paper is X mg/cm2 or greater, the slipping-out of the toner will occur unless the bias applied from a power supply (bias applying means) to the fur brush (cleaning means) is Vtr1 or greater. Consequently, the bias applied to the fur brush must be set to a bias of Vtr1 or greater.
As the result, in order to remove the untransferred toner in the indentation of the embossed paper, the cleaning bias must be made Vtr1 or greater even for other recording materials than the embossed paper in which the amount of secondary-transferring residual toner is less than X mg/cm2, and there arises the problem that the reversal of the charging polarity of the toner begins and the toner once collected by the fur brush is discharged onto the intermediate transfer belt.